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THE RAIL SHELLY CRACK IN JAPAN

Investigation into the growth mechanism of shelly cracks and a method of prevention is reported. Examinations included track condition, materials quality, loading condition and friction and wearing phenomenon of affected rails. It was found that in the ordinary contact condition, the contact stress between tyre and rail is so large that plastic flow is caused. With the progress of plastic deformation, steel becomes hard and its elastic limit rises. After these changes, rail surface becomes fit for ordinary wheel load. Though before this time the progress of plastic flow nearly stops, the crack grows through the fatigue due to rolling with slide. It was also found that the existence of ferrite in the surface layer seems to take part in the growth of the crack. As a first step in preventing shelly cracks, rail steel should be strengthened against fatigue. Water decreases the wear of rail and accelerates the development of crack. These conditions promote the growth of the crack due to the rolling fatigue. Secondly, though the decrease of stress is difficult to realize in general, the decrease of lateral force and the decrease of contact stress by decreasing the difference of both curvatures in contact part of wheel and rail are found effective. Thirdly, as it is clear that the existance of water is unfavourable, total stop or restraint of watering is very effective. Since the same effect is achieved by oiling, it is necessary to be careful not to oil excessively.